crash_test to cover small max_open_files (#6719)

Summary:
RocksDB behavior is different while max_open_files is small or large. Add the coverage to small max_open_files.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6719

Test Plan: Run crash_test

Reviewed By: pdillinger

Differential Revision: D21081021

fbshipit-source-id: e3e211761a9bd25d93d19a61c1f7b62d48cf5e3c

tools/db_crashtest.py

@@ -66,7 +66,7 @@ default_params = {
     "max_write_buffer_number": 3,
     "mmap_read": lambda: random.randint(0, 1),
     "nooverwritepercent": 1,
-    "open_files": lambda : random.choice([-1, 500000]),
+    "open_files": lambda : random.choice([-1, -1, 100, 500000]),
     "partition_filters": lambda: random.randint(0, 1),
     "pause_background_one_in": 1000000,
     "prefixpercent": 5,